1. Field of the Invention
The present invention relates to a favorable process for preparing 9-anthracenecarbaldehydes. 9-Anthracenecarbaldehydes are useful intermediates for preparing crop protection agents and pharmaceutics.
2. Discussion of the Background
DRP 519 444 discloses a preparation of 9-anthracenecarbaldehyde where anthracene is reacted with N-methylformanilide in the presence of phosphoryl chloride (=phosphorus oxychloride=POCl.sub.3) at 80.degree. C., the reaction mixture is subsequently introduced into dilute hydrochloric acid, and the product, after short heating, is precipitated and recrystallized from glacial acetic acid. Later, this process was improved by employing 1,2-dichlorobenzene as solvent (Org. Synth. Coll. Vol. III, p. 98 (1958)), giving rise to yields of up to 84%. This procedure, however, has various disadvantages. Thus, N-methylaniline which is obtained as byproduct has to be disposed of in an ecologically safe way, or has to be re-used in the preparation of N-methylformanilide and recycled. N-methylaniline and, if appropriate, 1,2-dichloro-benzene have to be removed by steam distillation. Since 9-anthracenecarbaldehyde is also volatile in steam, some of the product is lost. Finally, the black oil that is initially obtained has to be recrystallized for purification in any case.
When dimethylformamide was used instead of N-methylformamide, a yield of 62.5% was obtained with 1,2-dichlorobenzene as solvent, and a yield of 45% was obtained when dimethylformamide was also used as solvent (J. Am. Chem. Soc. 75, 989 (1953)), but again, the recrystallization of the product could not be dispensed with. Preparation of 9-anthracenecarbaldehyde in almost quantitative yield is possible by reacting anthracene with a trifluoromethanesulfonic anhydride/dimethylformamide complex. In this process, however, the sodium trifluoromethanesulfonate that is obtained has to be recovered and recycled into the anhydride for economical and ecological reasons.